The project deals with (1) changes in cognitive functioning over the adult lifespan and (2) effects of subject self-selection on scientific generalization in the aging literature. A carefully selected, age-varied sample of volunteer and nonvolunteer subjects performs a spectrum of intellectual tasks. Each subject is tested for 4 hours on metamemory, inference, word-knowledge recall and recognition, word recall and recognition, and several WAIS subtests. The research addresses the potential uninterpretability of psychological aging experiments due to restricted subject sampling. Most psychological studies of aging use volunteers in their older subject groups. We have found that in a population with known paramaters as little as 20% are willing to serve in a 2-hour experiment for $10. The 1 person in 5 who will serve may be cognitively unlike the 4 who refuse. Moreover, cognitive correlates of volunteerism may be different in different age groups. The current studies include two samples of subjects: those who have previously agreed to serve for $10 and those who will only serve for $80. The subject attrition rate is thereby reduced from 75% to 85% of the target population to around 20% to 25%. The project is designed for extrapolation to real-world functioning of young and old adults. Retrieval, inference, and metamemory are focal cognitive activities and are essential to functioning in the real world. Each of the three functions supports the actualization of permanently-stored knowledge of the world. The studies are designed to suggest stable periods, peaks and points of decline over the life-span in each of the three cognitive functions or systems.